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User blog:Granpa/Weather
Earth's weather is primarily driven by rising air in 3 low pressure areas. *The northern hemisphere polar front. **Extratropical cyclones form here and move eastward at 12-15 m/s (43-54 km/h) and last 3-5 days. (3600-6000 km) *The Intertropical Convergence Zone. (Equator) **Tropical cyclones form here due to convection and move westward. *The southern hemisphere polar front. **Extratropical cyclones form here and move eastward And to a lesser extent by descending air in 2 high pressure areas: *Northern hemisphere subtropical ridge (horse latitudes) *Southern hemisphere subtropical ridge (horse latitudes) Fronts The polar front is a cold front that arises as a result of cold polar air meeting warm subtropical air at the boundary between the polar cell and the Ferrel cell in each hemisphere. A cold front is the leading edge of a cold dense mass of air, replacing (at ground level) a warmer mass of air. Temperature changes across the boundary can exceed 30 °C (54 °F). Like a hot air balloon, the warm air rises above the cold air. The rising warm air expands and therefore cools. The Dry Adiabatic Lapse Rate (DALR) is a constant 0.98 °C for every 100 metres it rises. ((°C × 9/5) + 32 = °F). When the air reaches the condensation level the moisture within it condenses into droplets and releases the latent heat of condensation (2,260 Joules per gram of water) which causes the warm air to rise even further. The heat released slows the rate of cooling. The Moist Adiabatic Lapse Rate (MALR) varies with temperature but is typically about 0.5 °C for every 100 metres (50 °C per 10 km). If the warm air is moist enough (Vapour pressure of water at 104 °F = 0.0728 bar ≈ 47 grams of water per kg of air), rain can occur along the boundary. A narrow line of thunderstorms often forms along the front.﻿ Atmospheric convection Even in the absence of a cold front, the heating of the ground by the sun can produce thermals which become cumulus clouds which can if the air is unstable enough become thunderstorms (cumulonimbus). Over the ocean large masses of thunderstorms can become cyclones which can become hurricanes. See Tropical cyclogenesis A moist atmosphere (100% relative humidity) with an environmental lapse rate grater than the Moist Adiabatic Lapse Rate is required to force the atmosphere to be unstable enough for convection. The International Civil Aviation Organization (ICAO) defines an international standard atmosphere (ISA) with a temperature lapse rate of 6.49 °C/km from sea level to 11 km. On a summer day, net solar energy received at a lake reaches 15 MJ per square metre per day. The specific heat of dry air at 1 bar is 1 kJ per kg per °C. See Diurnal temperature variation and Planetary boundary layer. If 80% of the energy is used to vaporize water then evaporation = 0.49 cm/day. Over a 30 minute period a normal thunderstorm releases 1015 Joules of energy (440,000 m3 of water) equivalent to 0.24 megatons of TNT (ten times larger than the bomb over Nagasaki). A storm that lasted 24 hours would release 48 times as much energy (48 x 1015 Joules). A hurricane (a tropical cyclone) releases 52 x 1018 Joules/day equivalent to 1000 simultaneous non-stop thunderstorms. The average thunderstorm produces about 200,000 m3 of rain, but large storms can produce 10 times more rainfall. Severe storms :See also: Tornado emergency and Particularly Dangerous Situation Tropical air is far warmer than air outside the tropics and therefore holds far more moisture and as a result thunderstorms in the tropics are much taller. Nevertheless severe thunderstorms are not common in the tropics because the storms own downdraft shuts off the inflow of warm moist air killing the thunderstorm before it can become severe. However, the downdraft creates an Outflow boundary called a gust front that acts like a miniature cold front that can spawn new thunderstorms out ahead of the first. Sometimes a line of thunderstorms forms along the gust front. See also: Mesoscale convective complex. Severe thunderstorms (called supercells) tend to occur further north because of the polar jet stream. Polar jet streams are typically located near the 250 hPa (about 1/4 atmosphere) pressure level, or 7 to 12 km and are strongest in winter. The width of a jet stream is typically a few hundred kilometres or miles and its vertical thickness often less than five km. Speeds over 398 km/h have been measured. Each large meander, or wave, within the jet stream is known as a Rossby wave. The jet stream pushes against the top of the thunderstorm displacing the downdraft so that it can no longer shut off the inflow of warm moist air. As a result severe thunderstorms can continue to feed and grow for many hours whereas normal thunderstorms only last 30 minutes. Updrafts of 300 km/h are possible. (7000 J/kg = 300 km/h) If supercells track to the right or left of the mean wind, they are said to be "right-movers" or "left-movers," respectively. Supercells can sometimes develop two separate updrafts with opposing rotations, which splits the storm into two supercells: one left-mover and one right-mover. From Wikipedia:Convective available potential energy: Convective available potential energy (CAPE), is the amount of energy a parcel of air would have if lifted a certain distance vertically through the atmosphere. CAPE is effectively the positive buoyancy of an air parcel and is an indicator of atmospheric instability. (See also Lifted index.) CAPE is measured in joules per kilogram of air. CAPE is calculated by integrating vertically the local buoyancy of a parcel from the level of free convection (LFC) to the equilibrium level (EL). Extremely large values of CAPE can result in explosive thunderstorm development and severe storms. But if the jet stream is strong enough then severe weather and tornadoes can also develop in an area of low CAPE values. The surprise severe weather event that occurred in Illinois and Indiana on April 20, 2004 is a good example. There was strong low-level wind shear and although overall CAPE was weak (1000 J/kg), there was strong CAPE in the lowest levels (1-3 km) of the troposphere which enabled an outbreak of minisupercells producing large, long-track, intense tornadoes. See here for more information. See also: Landspout. Highest windspeed ever recorded was 480 km/h in the 1999 Bridge Creek–Moore tornado. The widest tornado on record is the El Reno, Oklahoma tornado of May 31, 2013 with a width of 4.2 km at its peak. A pressure deficit of 100 millibars was observed when a violent tornado near Manchester, South Dakota on June 24, 2003 passed directly over an in-situ probe that storm chasing researcher Tim Samaras deployed. In less than a minute, the pressure dropped to 850 millibars. The April 25–28, 2011 Super Outbreak was the largest, costliest and one of the deadliest tornado outbreaks ever recorded. In total, 360 tornadoes were confirmed by NOAA's National Weather Service (NWS) and Government of Canada's Environment Canada in 21 states from Texas to New York to southern Canada. Downdraft CAPE (DCAPE), estimates the potential strength of evaporatively cooled downdrafts. Tornado tracks in the US (1950-2017): Cyclones From Wikipedia:Aleutian Low: Cyclones (Hurricanes/Typhoons) that form in the tropical and equatorial regions of the Pacific normally start off by moving toward the west but can veer northward and get caught in the Aleutian Low (the polar front) where they become Extratropical cyclones which move toward the east. This is usually seen in the later summer seasons. Both the November 2011 Bering Sea cyclone and the November 2014 Bering Sea cyclone were post-tropical cyclones that had dissipated and restrengthened when the systems entered the Aleutian Low region. The storms are remembered as two of the strongest storms to impact the Bering Sea and Aleutian Islands with pressure dropping below 950mb in each system. The magnitude of the low pressure creates an extreme atmospheric disturbance, which can cause other significant shifts in weather. Following the November 2014 Bering Sea cyclone, a huge cold wave, November 2014 North American cold wave, hit the US bringing record breaking low temperatures to many states. The record lowest pressure established in the northern hemisphere is the extratropical cyclone of January 10, 1993 between Iceland and Scotland which deepened to a central pressure of 912-915 mb (26.93”-27.02”). Most hurricanes have an eye below 990 millibars. In 2005, hurricane WILMA reached the lowest barometric pressure ever recorded in an Atlantic Basin hurricane: 882 millibars. Hurricanes don't form in the South Atlantic. Tracks of all Tropical cyclones which formed worldwide from 1985 to 2005: Monsoons Air at the equator (Intertropical Convergence Zone) normally travels toward the west but the Indo-Australian monsoon causes so much air to rise over the Maritime Continent (See Tropical Warm Pool) that between Africa and the Maritime Continent the wind reverses direction and equatorial air travels eastward from Africa toward the Maritime Continent. The South American monsoon has a similar effect over the Pacific ocean west of Brazil. See Humboldt Current. The South Pacific convergence zone (SPCZ) & South Atlantic convergence zone (SACZ) are Monsoon troughs that branch off the The Intertropical Convergence Zone (ITCZ) at the points where the Indo-Australian monsoon and the South American monsoon occur. The Inter-Ocean Convergence Zone has traditionaly been called the Congo air boundary. Also called the South Indian Ocean Convergence Zone (SIOCZ) and Oceanic Tropical Convergence Zone (OTCZ). See also Asymmetry of the Intertropical Convergence Zone During an El Niño the South American monsoon is unusually strong and the Indo-Australian monsoon is weak. During an La Niña the opposite occurs. See Walker circulation. During a La Nina, a double ITCZ sometimes forms in the eastern Pacific, with one located north and another south of the Equator, one of which is usually stronger than the other. When this occurs, a narrow ridge of high pressure forms between the two convergence zones. The Madden–Julian oscillation is a traveling pattern that propagates eastward at approximately 4 to 8 m/s (14 to 29 km/h, 9 to 18 mph), through the atmosphere above the warm parts of the Indian and Pacific oceans. This overall circulation pattern manifests itself most clearly as anomalous rainfall. In the Pacific, strong MJO activity is often observed 6 – 12 months prior to the onset of an El Niño episode, but is virtually absent during the maxima of some El Niño episodes, while MJO activity is typically greater during a La Niña episode. Atmospheric rivers Extratropical cyclones can become so large that they draw moisture up directly from the tropics in what is called an atmospheric river. (See the image to the right.) Atmospheric rivers are typically several thousand kilometers long and only a few hundred kilometers wide, and a single one can carry a greater flux of water than the Earth's largest river, the Amazon.Wikipedia:Atmospheric river The Amazon discharges more water into the oceans than the next 7 largest rivers. See Zipf's law. (Like many other rivers the Amazon river valley is an Aulacogen.) During atmospheric river events both the Arctic oscillation and the Pacific–North American teleconnection pattern (a phase of the MJO) tend to be in the negative phase. The negative phase of the Arctic oscillation (AO) is associated with higher pressure in the Arctic and lower pressure in the surrounding lower latitudes. The negative phase of the Pacific–North American teleconnection (PNA) pattern features below-average pressure in the vicinity of Hawaii and over the inter-mountain region of North America, and above-average pressure south of the Aleutian Islands and over the southeastern United States. The negative phase tends to be associated with Pacific cold episodes (La Niña). Maintaining Atmospheric pressure If Earth's atmosphere were only slightly thicker then the air would be warmer and the amount of water vapor in the air would be much greater and lightning would therefore be much more common. The lightning would break apart the air molecules which would be washed down into the sea where they would end up in sediments which get subducted into the Earth. In this way the Earth's average air pressure is maintained at its current level. During most of it's history Earth only had one atmospheric cell that extended from the pole to the equator and as a result Earth was very much warmer. #Hadley cell During an ice_age the Earth only has two cells. Ice ages are probably caused by deforestation caused by megafauna. #Polar cell #Ferrel cell﻿ The Earth's atmosphere currently has 3 cells. #Polar_cell #Ferrel_cell #Hadley_cell﻿ Misc images Blue represents rising air; Red represents sinking air: References External links *precipitable water *Madden-Julian Oscillation monitoring *Weekly mjo update *National Weather Service *Meteorology for Scientists and Engineers *AAO, AO, NAO, PNA *https://svs.gsfc.nasa.gov/4285 *https://eldoradoweather.com/climate/world-maps/world-snow-ice-cover.html *https://climate.rutgers.edu/snowcover/ Category:Blog posts